Utility of MPT64 antigen test for differentiating mycobacteria: Can correlation with liquid culture smear morphology add further value.

ABSTRACT

Context Clinical presentation of Mycobacterium tuberculosis complex (MTBC) and non‑tuberculous mycobacteria (NTM) infections may or may not be the same, but the treatment is always different. Hence accurate differentiation between MTBC and NTM is of utmost importance. Aims: To assess in parallel, the utility of MPT64 antigen immunochromatography assay (MPT64 ICT) and bacillary morphology on liquid culture smear, for rapid differentiation between MTBC and NTM in clinical isolates. Settings and Designs Private sector reference laboratory, prospective. Subjects and Methods: Thousand and ninety‑three mycobacterial isolates, recovered using Mycobacteria Growth Indicator Tube 960 liquid culture system (BD, USA), were subjected to MPT64 ICT (Standard Diagnostics Inc., Korea), para amino nitrobenzoicacid (PNB), niacin, and nitrate reduction tests. Smears prepared from culture vials were subjected to Ziehl‑Neelsen staining and observed microscopically for typical patterns (chords, single cells, etc.,). PNB, nitrate and niacin tests served as the reference method for MTBC identification. Results: Thousand and fourteen and 79 isolates were identified as MTBC and NTM, respectively. MPT64 ICT correctly identified 955/1014 MTBC and all NTM isolates, yielding sensitivity and specificity of 94.2% and 100%, respectively. 936/1014 (92.3%) MTBC isolates revealed characteristic serpentine chording on culture smear including 56/59 MPT64 ICT negative isolates. Sensitivity and specificity of liquid culture smear were 98.1% and 82.3%, respectively. Conclusion: Correlation of MPT64 ICT results with liquid culture smear was useful, especially in MPT64 ICT negative isolates, where the latter could help to determine need and/or type of additional confirmatory testing. Liquid culture smear, however, lacked specificity and cannot be used as a stand alone test.